The goal of the proposed ancillary studies is to establish whether characterization of the insulin-glucagon-glucose (IGG) interactions in first degree relatives of patients with type 1 diabetes (T1D) can provide new information about the pathogenesis, prediction, and progression of the early stages of the disease. The project will enroll individuals from the Living Biobank of the TrialNet Pathway to Prevention (PTP) study who are phenotyped with respect to a variety of risk factors, including immunological abnormalities. To the best of our knowledge, the IGG relationships in general and the glucagon phenotype in particular have not been studied in this population. It is known, however, that in T1D the release of glucagon is altered, which is manifested by abnormal postprandial suppression and defective response to hypoglycemia. Several reports indicate that glucagon becomes dysregulated prior to the development of T1D, but comprehensive studies aiming to understand in detail the insulin-glucagon co-dynamics in people at risk for T1D have never been performed. Thus, our goals now are to transfer our expertise in clinical testing and analysis of the IGG interactions and to expand our methodology to characterize the IGG interactions in individuals at risk for developing T1D.To achieve our goals, we will first quantify the primary intra-islet interactions using our Minimal Control Network models of glucagon secretion and counterregulation. Further, our preliminary data show that in patients with T1D, continuous glucose monitoring (CGM) field data provide information about the patient's insulin sensitivity, and glucagon and epinephrine counterregulation. Accordingly, our second goal is to test whether a simple minimally invasive test performed in the field can estimate the state of the IGG interactions. We hypothesize that in first degree relatives of patients with T1D, immunological abnormalities are associated with alterations in the interactions between blood glucose, insulin and glucagon which can be: (i) detected with a combined 10-hour mixed-meal and induced hypoglycemia inpatient test and (ii) correlated with self-collected minimally-invasive CGM field data. The following Specific Aims are proposed for studies in first degree relatives of patients with T1D enrolled in (or screened for participation in) the T1D TrialNet clinical trials network. Aim 1 is to test the hypothesis that immunological abnormalities are associated with abnormally high glucagon responses to a meal and reduced glucagon counterregulatory response to insulin induced hypoglycemia. Aim 2 is to correlate metrics derived from a minimally-invasive CGM-based field test with glucagon responses to a meal and induced hypoglycemia measured in the hospital. When completed, this study will improve the understanding of the pathogenesis of the early stages of T1D and will provide new quantitative tools for prediction and evaluation of insulin-glucagon-glucose interactions relevant to individuals at risk for developing T1D, thereby enabling future preventive intervention trials.